High-precision sensor circuit for small-diameter cylindrical contact displacement detector

ABSTRACT

A displacement detector has a contact sensor incorporating a differential transformer and a circuit for adjusting the amplitude of the driving signal for driving a differential transformer of the sensor by feeding back as a standard signal the driving signal through an amplifier and an AC-DC converter as a standard signal. The sensor has a housing containing a mobile member. The mobile member has a shaft with a hole. An outer tubular body of a linear bush and a ball guide each have a hole, and a pin is inserted through these holes movably in the direction of movement of the mobile member such that the rotation of the mobile member is prevented.

BACKGROUND OF THE INVENTION

This invention relates to a high-precision sensor circuit for a contactdisplacement detector for accurately measuring dimensions and shape ofdevice components or assemblies at a factory, and more particularly to acontact displacement sensor incorporating a differential transformer anda circuit for forming a displacement detector incorporating such asensor.

FIG. 28 shows the circuit structure of a prior art contact displacementdetector with a sensor 201 incorporating a differential transformer 202and a sensitivity-adjusting resistor 203. The differential transformer202 has a mobile core (not shown) and two coils 204 and 205 disposedaround this mobile core. These two coils 204 and 205 are connected inseries and driven by an AC voltage provided as a driving signal from anoscillator 206 through an amplifier 207. Output signals are taken outfrom a junction point in between.

This displacement detector is a transducer of the half-bridge type. Theinductance of the two coils 204 and 205 driven by an AC voltage is afunction of the position of the mobile core. The inductive voltagesgenerated in the two coils 204 and 205 are equal to each other if themobile core is at the center of the two coils 204 and 205. If the mobilecore is displaced from this center position, the inductive voltage ofone of the coils 204 or 205 increases and that of the other coil 204 or205 decreases. A contact member (not shown) for contacting the targetobject of measurement is attached to this mobile core and the sensor isadapted to detect the displacement of this contact member.

The output signal from the junction at the center of the two coils 204and 205 is an AC output of which the amplitude changes according to thedisplacement of the mobile core. After being amplified by an amplifier208, this output AC signal is subjected to a full-wave rectificationprocess by an AC-DC converter 209 and inputted to the non-inversioninput terminal of a differential amplifier 210. Another AC voltageapplied from amplifier 207 to the differential transformer 202 isinputted to the inversion input terminal of this differential amplifier210 through amplifier 211 and AC-DC converter 212 to serve as a standardsignal. The differential amplifier 210 amplifies the standard signal andthe output signal from the differential transformer 202 differentiallyand outputs a signal corresponding to the displacement of the mobilecore.

According to this illustrated example, not only is the sensor 201 itselfprovided with a sensitivity-adjusting resistor 203, but the amplifier208 for amplifying the output signal from the differential transformer202 is provided with a gain-switching resistor 213 such that the gain ofthe amplifier 208 can be changed, depending on the kind of the sensor201, that is, such that the same circuit can be used with sensors ofdifferent kinds with different ranges of measurement (or strokes).

According to this example, furthermore, a pull-down resistor 214 isconnected to the output signal line of the differential transformer 202and there is also provided a comparator 215 for comparing the outputfrom the AC-DC converter 209 with a threshold value to provide adetection output. If there is a breakage in the sensor cable connectedto the sensor 201, or when the wire for transmitting a signal fordriving the sensor 201 is broken (as indicated by A1) or the sensorsignal output line A2 is broken, for example, the AC voltage signaloutputted from the sensor 201 is not communicated and becomes zero bythe pull-down resistor 214 such that the breakage can be detected by thecomparator 215. If the breakage is only in the grounding line, as shownby A3, the sensor driving signal is not divided by the coils 204 and 205and hence the sensor driving signal is directly outputted. This, too,can be detected by the comparator 215.

For carrying out measurements with a high level of accuracy with such aprior art sensor, very small signals from the differential transformermust be taken out at a high level of stability and with a high S/Nratio. Moreover, the output from the amplifier 207 to become a standardsignal must also be stable. For this purpose, an oscillator and anamplifier such as an operational amplifier with high accuracy andstability are required. For obtaining a high S/N ratio and stability, adedicated IC incorporating an operational circuit for temperaturecompensation, etc. must be used, and this affects the production costadversely.

Since different sensors have different sensitivities, furthermore, thegain of the amplifier 208 is adjusted by means of the gain-switchingresistor 213. Thus, if a sensor with low sensitivity is used, the S/Nratio becomes lowered as the gain is increased. Although it is desirableto use processing systems having similar processing characteristics forthe standard signal and the output signal from the differentialtransformer 202, the processing system for the output signals from thedifferential transformer 202 is different from that for the standardsignal, being adapted to switch to change the gain. Thus, it isdifficult to make the temperature characteristics of the componentsuniform and to place the components in a thermally well balanced manner.

Moreover, since the breakage of the sensor cable is detected on thebasis of the output AC signal, if the inductance of the differentialtransformer 202 is increased in order to improve the sensitivity of thesensor 201, the output AC signal from the differential transformerbecomes unstable due to the capacitive coupling between the signal linesat both ends of the coil 204 or 205 when there is a breakage in thesensor cable and the breakage may not be detected dependably. It may beattempted therefore to reduce the resistance of the pull-down resistor214 in order to reduce the effect of the capacitive coupling but if theresistance of the pull-down resistor 214 is reduced, the linearitycharacteristic of the differential transformer 202 becomes adverselyaffected. A similar result is obtained even if a pull-up resistor isused instead of the pull-down resistor.

Another problem of prior art displacement sensors of this kind relatesto their structure. If the diameter of a sensor is reduced from φ8 toφ6, for example, the sensor can be attached to a target object (such asa machine) more intimately and the target object can be made morecompact. Since the weight of the mobile parts of the sensor must bereduced accordingly, the load to the sensor can be reduced and hence thesensor becomes usable for the measurement of an object which could notbe measured because of its large load. When the diameter of a sensor isreduced from φ8 to φ6, however, it is not sufficient to merely reduceits linear dimensions to three quarters (0.6/0.8) of the original and toreduce the cross-sectional area by a factor of (0.6/0.8)²=0.56. Itcannot be ignored that stoppers for the rotation of a mobile componentfor driving the core member, for example, must retain their originalfunction and capability. Moreover, the difficulty in assembly because ofreduced size of components must be considered and the need forwater-proofing between the mobile components for the core member becomesmore important.

FIG. 23 shows the structure of an example of prior art displacementsensor, having a linear bush 81 and the bobbin assembly of adifferential transformer 95 inside a housing 80. A mobile member 101having a mobile shaft 91 and a core member 89 connected to this mobileshaft 91 is movable longitudinally inside this housing 80 through alinear bush 94. The core material 89 is inserted into the bobbinassembly of the differential transformer 95 to form the differentialtransformer 95. The mobile member 101 is biased by means of a springmember (not shown) such as a parallel coil spring with invariable coildiameter so as to protrude the tip of the mobile shaft 91 out of thehousing 80 and a contact member 93 is formed at the protruding portionof the mobile shaft 91. The linear bush 94 is of a structure havinginserted inside an outer tubular body 81 with an elongated hole 88 onits circumference a ball guide 84 with many balls 84 a held on itscircumference. A rotation-preventing pin 92 on the shaft 91 is insertedinto the elongated hole 88 of the outer tubular body 81 so as to stopthe rotation of the mobile member 101.

FIG. 24A shows another prior art displacement sensor, having a linearbush 94 and the bobbin assembly of a differential transformer 95 insidea housing 80. A mobile member 101 having a mobile shaft 91 and a coremember 89 connected to this mobile shaft 91 is movable longitudinallyinside this housing 80 through the linear bush 94. The core material 89is inserted into the bobbin assembly of the differential transformer 95to form the differential transformer 95. The mobile member 101 is biasedby means of a spring member (not shown) such as a parallel coil springwith invariable coil diameter so as to protrude the tip of the mobileshaft 91 out of the housing 80 and a contact member 93 is formed at theprotruding portion of the mobile shaft 91. A rotation-preventing member102 on the mobile shaft 91 has a groove 102A formed extending in theaxial direction of the housing 80. A metallic rotation-preventing guidepin 103 is pressed into a hole 80 a in the housing 80 and into thegroove 102A as shown in FIG. 24B to prevent the rotation of the mobilemember 101.

FIG. 25 shows the structure for leading a cable out of the housing 80 ina sealed manner, including a cable-stopping member 110 having aresin-filling portion 108 and a cable-passing opening part 109. After acable 96 is inserted into the opening part 109, an O-ring 111 is placedbetween the resin-filling portion 108 and the opening part 109, and theresin-filling portion 108 is filled with an epoxy resin material 112,and the cable-stopping member 110 is pressed into the backward end ofthe housing 80.

With a prior art displacement sensor structured as shown in FIG. 23, therotation-preventing pin 92 is inserted into the elongated hole 88 on theouter tubular body 81 of the linear bush 94 in order to prevent therotation of the mobile member 101. Thus, the elongated hole 88 must beformed on the outer tubular body 81 and burrs are left on the innersurface of the outer tubular body rubbed by the balls 84 a. A workprocess for removing these burrs becomes necessary, and the constructionand preparation of this linear bush becomes complicated. Such means forpreventing the mobile member from rotating cannot be used directly witha displacement sensor as the latter is becoming miniaturized because theworkability efficiency will be significantly affected. In particular,the distance of the displacement in the direction of the motion of themobility becomes increased and the outer tubular body 81 of the linearbush 94 comes close to the differential transformer 95. Thus, themagnetic property of the outer tubular body 81 may come to adverselyaffect the characteristics of the differential transformer 95, causingthe product quality and temperature characteristic to become unstable.

With a prior art displacement sensor structured as shown in FIG. 24Aadapted to have a metallic guide pin 103 inserted into a groove, the pin103 will slide inside the groove 102A as the mobile member 101 is moved,and this increases the friction between metals, making it difficult toreduce the force required for the operation. Since the housing 80 has ahole 80 a for accepting the pin 103, water-proofing cannot be made andthe device diameter cannot be reduced because the portion around thehole 80 a must be made sufficiently thick. Additional problems are thatthe length in the mobile direction increases and the production cost ofthe housing 80 becomes higher. Since a parallel coil spring withinvariable coil diameter is used for the mobile shaft 91, furthermore,the coil will rub against the neighboring components to cause frictionand interference. Moreover, since the cable is fastened to the housing80 by passing the cable 96 through the opening part 109 of thecable-stopping member 110, placing the O-ring 111 between theresin-filling portion 108 and the cable-passing opening part 109,filling the resin-filling portion 108 with the epoxy resin 112 andpressing the cable-stopping member 110 into the back end of the housing80, there is a large variation in the strength and the cable cannot bekept flexible.

FIG. 26 shows still another prior art displacement sensor providing ahousing 80-1 with a female screw part 120 and an outer tubular body 81-1of a linear bush 94 with a male screw part 121, It is assembled with themale screw part 120 engaged with the female screw part 121 to tighten aflat packing member 122 in a watertight manner. Since it requires aheight corresponding to the ridge portion of the male screw part 121,the sensor is prevented from being made compact. An adhesive may be usedinstead of screws, but this leaves the problem of dependability in thesealing.

FIG. 27 shows still another prior art displacement sensor assembled byinserting a rubber boot 123 onto a mobile shaft 125 from the side of ameasurement piece 124. Since the mobile shaft 125 is provided with amale screw part 126 for attaching the measurement piece 124, the innersurface (sealing surface) of the sealing part 123 a of the rubber boot123 is easily damaged, and this again leaves the problem ofdependability in the sealing.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a displacementdetector capable of detecting displacements accurately.

It is another object of this invention to provide such a displacementdetector which can be produced inexpensively.

It is still another object of this invention to provide a displacementdetector capable of dependably detect a wire breakage.

It is therefore an object of this invention to provide a contactdisplacement sensor structured so as to be made compact whilemaintaining its original functions and capabilities.

A displacement detector embodying this invention may be characterized ascomprising a differential transformer, driver means for generating adriving signal for driving the differential transformer, standard signalprocessing means for processing the driving signal and therebyoutputting a standard signal, output signal processing means forprocessing signals outputted from the differential transformer,differential amplifier means for carrying out differential amplificationof the standard signal and the output signal from the output signalprocessing means, and amplitude adjusting means for adjusting theamplitude of the driving signal to a constant value by feeding back thestandard signal to the driver means. With a displacement detector thuscharacterized, a stable driving signal can be obtained for driving thedifferential transformer and a stable standard signal can be provided tothe differential amplifier means. This is unlike a prior artdisplacement detector requiring expensive, highly stable components suchas oscillator and amplifiers because an open-loop signal routine wasemployed.

Since the amplitude of the driving signal for driving the differentialtransformer is adjusted according to the kind of the differentialtransformer, or the sensitivity of the differential transformer, the S/Nratio does not drop as in the case of a prior art detector adapted toadjust the gain. Since the gain is not switched, the signal processingby the standard signal processing means and the output signal processingmeans can be made equal. As a result, the components can be arranged ina thermally balanced manner.

Since the amplitude adjusting means adjusts the amplitude of the drivingsignal such that it will take upon a value corresponding to theaforementioned standard value, the standard signal and the standardvalue become nearly equal, and similar merits as described above canresults if the standard value instead of the standard signal is given tothe differential amplifier means.

According to another embodiment of the invention, the standard signalprocessing means and the output signal processing means each comprise anamplifier circuit and an AC-DC converter, and at least either theseamplifiers or these AC-DC converters are thermally coupled, for example,by being packaged together or by being placed appropriately. Thus, notonly temperature variations in the standard signal processing means areautomatically corrected according to this invention because the standardsignal is fed back, temperature variations also become alike in thestandard signal processing means and the output signal processing meansbecause they are thermally coupled. Their variations can be cancelledtogether by the differential amplifier means on the downstream side, anda highly accurate detection becomes possible.

According to still another embodiment of the invention, an abnormalcondition of the detector is detected on the basis of the level of a DCbias which is superimposed to the output signal from the differentialtransformer. In the case of a breakage in the sensor cable, the level ofthe DC bias superimposed to the output signal from the differentialtransformer becomes outside a specified range, and this makes itpossible to detect a breakage in the cable. Even if the inductance ofthe differential transformer is high, such an abnormality can bereliably detected without being affected by the thermal coupling, andthere is no need to reduce the resistance of a pull-down resistor or apull-up resistor. A trouble in the driver means can also be detectedsimilarly.

A displacement sensor embodying this invention may be characterized ascomprising a linear bush and a mobile member inside a housing androtation-preventing means for preventing rotation of the mobile member.The linear bush includes an outer tubular body extending in its axialdirection and containing a holder which is movable in the axialdirection of the outer tubular body. The mobile member has a mobileshaft supporting the core member of a differential transformer and ismovable in the same axial direction, being biased outwardly by a spring.The outer tubular body of the linear bush and the holder inside theouter tubular body are each provided with a hole, and the mobile shaftof the mobile member includes a pin-accepting hole part. Therotation-preventing means comprises a rotation-preventing member such asa pin which is inserted movably through these holes in the outer tubularmember and the holder and into the pin-accepting hole part. The outertubular body of the linear bush is a tubular member to be attached tothe inner surface of the housing when the linear bush is engaged withthe housing and adhesively attached to the housing. The holder maycomprise a ball guide holding many balls thereon.

With a structure as described above, the rotation-preventing means canbe contained inside the main body of the sensor such that the sensor canbe made compact and shorter. It also helps to increase the strength ofthe mobile shaft and its production becomes easier.

In another aspect of the invention, the housing has protrusions formedthereon, protruding in the inward direction towards its interior, eachprotrusion has a stopping surface perpendicular to the axial directionof the sensor, and the housing includes a stopper having an outersurface with flat parts and a contact surface which is at one end ofthese flat parts and is also perpendicular to the axial direction. Theseprotrusions are positioned at the flat parts around the stopper and thecontact surface and the stopping surface contact each other to positionthe stopper and to prevent the stopper from rotating. With the sensorthus structured, the stopper can be affixed to the housing, positionedand prevented from rotating as the stopper is inserted inside thehousing with the protrusions positioned at the flat parts and thestopping surface and the contact surfacing contacting each other.

These protrusions are produced according to this invention by punchingthe housing inward by the so-called “punch-stretch forming method” andgrinding its outer surface areas in a centerless grinding process toreduce the thickness of the housing while maintaining a specified amountof protrusion. By such a method, protrusions with a specified height canbe produced even if the material of the housing is relatively thick andsince the housing is made thinner, the sensor can be made more compactaccordingly.

The displacement sensor embodying this invention may be so structuredthat both the housing and the mobile member have two (first and second)stopper parts for preventing the aforementioned rotation-preventingmember from hitting a near-by component and becoming thereby deformed.The stopper parts are so positioned that as the spring is stretched asmuch as possible until the first stopper parts come to contact eachother (the spring being at the “stretched limit position”), there is afinite interval between the rotation-preventing member and one of theend parts of the holes into which the rotation-preventing member isinserted and that as the spring is pushed in and contracted as much aspossible until the two second stopper parts come to contact each other(the spring being then at the “pushed-in position”), there is similarlyanother finite interval between the rotation-preventing member and theother of the end parts of the same holes. These two first stopper partsmay be formed respectively on the outer tubular body of the linear bushand the stopper, and the first and second stopper parts of the mobilemember may be formed on a core shaft. With stopper parts thus formed,the rotation-preventing means does not hit either the front or back endpart of the elongated hole in which it slides as the mobile member ismoved by stretching or contracting the spring in either direction. Thus,the rotation-preventing means is not deformed.

According to a preferred embodiment, a conic coil spring is used,supported between the stopper and the core shaft and the core shaftincludes a tapered part for avoiding interference with this conicspring. Thus structured, the conic spring is not interfered byneighboring components even if the sensor is made compact as a whole.

The mobile shaft and a holder for the contact member (the“contact-member holder”) may be realized as separate components, and arubber boot is attached by engaging its front and back end partsrespectively with a front boot holder on the contact-member holder and aback boot holder on an end cap with which the outer tubular member isprovided. The contact-member holder is thus attached to the rubber boot,and the contact-member holder is connected to the rubber boot. In thismanner, the inner surface of the rubber boot is not damaged and the endcap makes the structure even more reliably watertight.

Where a cable is connected to the sensor, a cable cap of a syntheticresin material is integrally formed with the cable and engaged with aback end part of the housing such that the cable is pulled out of theback end part of the housing. Polyester elastomer may be used for thispurpose. Since the cable-holding part thus formed is not a separatecomponent of the sensor, the total number of the constituent parts isreduced and the production cost can also be reduced. Furthermore, thecable can be made more flexible.

In order to attach the cable cap to the housing in a watertight manner,a groove is formed on the cable, filled with an adhesive. A protrusionis formed in the groove such that it will contact the inner surface ofthe housing as the cable cap is attached to the housing and the adhesiveis sealed inside.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a displacement detector embodying theinvention.

FIGS. 2, 3 and 4 are circuit diagrams of other displacement detectorsembodying the invention.

FIG. 5 is a circuit diagram of portions of the AC-DC converters of FIG.4.

FIG. 6 shows thermally coupled operational amplifiers.

FIG. 7 shows a thermally coupled diode array.

FIG. 8 is an exploded diagonal view of a displacement sensor embodyingthis invention.

FIG. 9 is a vertical sectional view of the displacement sensor of FIG.8.

FIG. 10 is a sectional view of a tip part of the displacement sensor ofFIG. 8.

FIG. 11 is a sectional view of a center part of the displacement sensorof FIG. 8.

FIG. 12 is a side view of a core shaft of the displacement sensor ofFIG. 8.

FIG. 13 is a sectional view of a backward part of the displacementsensor of FIG. 8.

FIG. 14 is an exploded diagonal view of the rotation-preventing means.

FIG. 15 is an exploded diagonal view for showing how the rubber boots ofthe displacement sensor of FIG. 8 is assembled.

FIG. 16 is a sectional view of a stopper on the housing.

FIG. 17 is a sectional view of a stopper developing a crack.

FIG. 18 is a plan view of the hole for stopping rotation in thedisplacement sensor of FIG. 8.

FIGS. 19A and 19B are plan views of holes not according to the presentinvention for stopping rotation.

FIG. 20 is a sectional view of a portion of the rubber boot for showingthe mechanism for its attachment.

FIG. 21 is a sectional view of a prior art mechanism for attaching arubber boot.

FIGS. 22A is a sectional view of the displacement sensor of thisinvention when the mobile member is at the stretched limit position, and22B is a sectional view of the displacement sensor of this inventionwhen the mobile member is at the pushed-in position.

FIG. 23 is an exploded diagonal view of a prior art displacement sensor.

FIG. 24A is an exploded diagonal view of another prior art displacementsensor and FIG. 24B is its sectional view.

FIG. 25 is a sectional view of the sealing structure of the cableportion of a prior art displacement sensor.

FIG. 26 is a diagonal view of a portion of a prior art displacementsensor where its housing and linear bush are connected.

FIG. 27 is a diagonal view of a prior art displacement sensor forshowing the assembly of its rubber boot.

FIG. 28 is a circuit diagram of a prior art displacement detector.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of this inventions are described next with reference todrawings.

FIG. 1 shows the structure of a displacement detector 217 embodying theinvention. Some of the components of this detector 217 are substantiallythe same as those explained above with reference to FIG. 28 and will beindicated by the same numerals for the convenience of disclosure.

In FIG. 1, numeral 201 again indicates a sensor forming the displacementdetector 217 of this invention, connected through a sensor cable andincluding a differential transformer 202 and a sensitivity-adjustingresistor 203. The differential transformer 202 has a mobile core (notshown) and two coils 204 and 205 disposed around this mobile core. Thesetwo coils 204 and 205 are connected in series and driven by an ACvoltage provided as a driving signal from an oscillator 206 through anamplifier 207. Output signals are taken out from a junction point inbetween. The oscillator 206 and the amplifier 207 together form a“driver means” for driving the differential transformer 202.

This displacement detector 217 is a transducer of the half-bridge type.The inductance of the two coils 204 and 205 driven by an AC voltage is afunction of the position of the mobile core. The inductive voltagesgenerated in the two coils 204 and 205 are equal to each other if themobile core is at the center of the two coils 204 and 205. If the mobilecore is displaced from this center position, the inductive voltage ofone of the coils 204 or 205 increases and that of the other coil 204 or205 decreases. A contact member (not shown) for contacting the targetobject of measurement is attached to this mobile core for detecting thedisplacement of this contact member.

The output signal from the junction at the center of the two coils 204and 205 is an AC output of which the amplitude changes according to thedisplacement of the mobile core. After being amplified by an amplifier208, this output AC signal undergoes a full-wave rectification processby an AC-DC converter 209 and is inputted to the non-inversion inputterminal of a differential amplifier 210. The amplifier 208 and theAC-DC converter 209 together form an “output signal processing means”for processing the output signals from the differential transformer 202.

Another AC voltage applied from amplifier 207 to the differentialtransformer 202 is inputted to the inversion input terminal of thisdifferential amplifier 210 through amplifier 211 and AC-DC converter 212to serve as a standard signal. The amplifier 211 and the AC-DC converter212 together form a “standard signal processing means” for outputtingthe standard signal by processing the driving signal.

The differential amplifier 210 amplifies the standard signal and theoutput signal from the differential transformer 202 differentially andoutputs a signal corresponding to the displacement of the mobile core.

In this example, not only is the sensor 201 itself provided with asensitivity-adjusting resistor 203, but the amplifier 208 for amplifyingthe output signal from the differential transformer 202 is provided witha gain-switching resistor 213 such that the gain of the amplifier 208can be changed, depending on the kind of the sensor 201, that is, suchthat the same circuit can be used with sensors of different kinds withdifferent ranges of measurement (or strokes). In this example,furthermore, a pull-down resistor 214 is connected to the output signalline of the differential transformer 202 and there is also provided acomparator 215 for comparing the output from the AC-DC converter 209with a threshold value to provide a detection output.

Unlike the prior art displacement detector described with reference toFIG. 28, the example of displacement detector 217 shown in FIG. 1includes an amplitude adjusting means 216 for adjusting to make uniformthe amplitude of the AC voltage for driving the differential transformer202 by feeding back the standard signals from the AC-DC converter 212 ofthe standard signal processing means to the driver means. This is forthe purpose of making it possible to detect displacement with a highdegree of accuracy and also stabilizing the output from the amplifier207 serving as the standard signal for the differential amplifier 210even if expensive kinds of oscillator, amplifier and dedicated IC arenot used. This amplitude adjusting means 216 stores a standard valueinternally and includes an amplitude adjusting circuit for adjusting theamplitude of the AC voltage from the oscillator 206 such that thestandard signal from the AD-DC converter 212 will approach this standardvalue.

With the amplitude of the driving signal provided to the differentialtransformer 202 thus maintained at a constant value by a feedbackcontrol, not only can the differential transformer 202 be operated by astable driving signal, the standard signal provided to the differentialamplifier 210 is also stabilized. Thus, according to this invention,accurate detection is possible even with an inexpensive displacementdetector. This is in contrast to prior art displacement detectors whichprocess signals by an open-loop routine and hence require accurate andhighly stable oscillator and amplifier such as operational amplifier.Even the effects of temperature variations are only on the standardvalue of the amplitude adjusting circuit and hence there is no need toadditionally provide any circuits for temperature compensation.

FIG. 2 shows the structure of another displacement detector 217 aembodying the invention, indicating like components by the same numeralsas in FIG. 1. Unlike the first embodiment characterized as switching thegain of the amplifier 208 by means of the gain-switching resistor 213according to the kind of the sensor 201, the second embodiment ischaracterized as switching the standard voltage of an amplitudeadjusting means 216 a according to the kind of the sensor 201 a.

Explained more in detail, the sensor 201 a in this example is providedwith a sensitivity-adjusting resistor 203 a, separate from thedifferential transformer 202, such that the divided voltage by thissensitivity-adjusting resistor 203 a and another resistor 218 isprovided to an amplitude-adjusting means 216 a as the standard voltage.This standard voltage and the standard signal from the AC-DC converter212 are compared by an operational amplifier 219, and anamplitude-adjusting circuit 220 adjusts the amplitude of the drivingsignal such that this difference will disappear.

In summary, according to the second embodiment of the invention, theamplitude of the driving signal to the differential transformer 202 isadjusted according to the kind of the sensor 201 a, that is, thesensitivity of the sensor 201 a. Thus, the change in the S/N ratio isreduced, and a higher S/N ratio can be obtained even with a sensor witha low sensitivity than by switching the gain of the amplifier.

Since the gain-switching resistor 213 of FIG. 1 is dispensed with, theprocessing by the output signal processing means with the amplifier 208and the AC-DC converter 209 become equivalent to that by the standardsignal processing means with the amplifier 11 and the AC-DC converter212. This makes it easier to regulate the temperature characteristics ofthe components or to arrange the components in a thermally balancedmanner. As will be explained below, variations in these means may bemade equal such that they can be cancelled together by means of thedifferential amplifier 210. This can further improve the accuracy ofdisplacement detection. In the above, the sensitivity-adjusting resistor203 and the resistor 218 for dividing voltage may be disposed in theconnector for connecting the sensor or in the detector 217 a. In otherrespects, the second embodiment is the same as the first embodiment.

Since the amplitude-adjusting means 16 a functions so as to adjust theamplitude of the driving signal and to make the difference disappearbetween the standard voltage and the signal from the AC-DC converter212, the standard voltage and the standard signal from the AC-DCconverter 212 become equal. Thus, as a variation, the standard voltagemay be inputted to the differential amplifier 210 instead of thestandard signal from the AC-DC converter 212, as shown in FIG. 3.Similarly, with reference to FIG. 1, the internal standard voltage ofthe amplitude adjusting means 216 may be inputted to the differentialamplifier 210 instead of the standard signal from the AC-DC converter212.

FIG. 4 shows the structure of still another displacement detector 217 b,indicating like components by the same numerals. This embodiment ischaracterized as superimposing a DC bias onto the AC voltage from theoscillator 206 to become the driving signal by means of a DC biassuperimposing circuit 221, using a low pass filter 222 to separate theDC bias superimposed onto the output signal from the differentialtransformer 202 given from the amplifier 208, and detecting a breakagein the sensor cable or a trouble in the driving means by using thecomparator 215 a on the basis of the DC bias level. This is in contrastwith the embodiments described above adapted to detect a breakage in thesensor cable, etc. on the basis of the AC output signal from thedifferential transformer 202. In the example illustrated, a detectormeans for detecting an abnormality is comprised of the low pass filter222 and the comparator 215 a.

By this example, a breakage in the sensor cable, for example, can bedetected by the comparator 215 a because the level of the DC biassuperimposed onto the output signal from the differential transformer202 goes out of a specified range. If a breakage occurs at A1 on theside of the sensor driving signal or at A2 on the sensor output signalline, the sensor output with the DC bias superimposed is notcommunicated and it becomes 0V because of the pull-down resistor 214.The breakage can thus be detected by the comparator 215 a. If thebreakage is only on the GND side of the sensor as indicated by A3, thesensor driving signal is not divided by the coils 204 and 205 and isdirectly outputted. The level of the DC bias becomes higher than aspecified level, and this is detected by the comparator 215 a. If thedriver means develops a trouble, too, the level of the DC biassuperimposed onto the output signal becomes outside a specified rangesuch that the trouble can be detected.

Since a feed-back control is carried out, like the examples describedabove, a control is effected when there is an abnormality developed inthe driver means and its output changes such that this change will becorrected. Although troubles in the driver means have been difficult todetect, the level of the superimposed DC bias is monitored according tothis example such that such an abnormality can also be detected as anaccident.

If the inductance of the differential transformer 202 is increased in aprior art example, the AC output signal from the differentialtransformer 202 becomes unstable when there is a breakage in the sensorcable due to the capacitive coupling C developed between the signallines at both ends of the coil 204 or 205 as shown in FIG. 4. In thepresent example, by contrast, there is no effect of such capacitivecoupling because the breakage is detected on the basis of the DC biasand hence the breakage can be detected reliably. Thus, there is no needto decrease the resistance of the pull-down resistor 214 and thelinearity characteristic of the differential transformer 202 is notadversely affected. The same holds when a pull-up resistor is usedinstead of the pull-down resistor 214.

In this example, like the second embodiment described above, the thermalcharacteristics of the components of the output signal processing meanscomprising the amplifier 208 and the AC-DC converter 209 and that of thestandard signal processing means comprising the amplifier 211 and theAC-DC converter 212 are matched, their components are arranged by takingtheir thermal balance into consideration, or thermal couplers are usedsuch that variations in these processing means will become equal andthey can be cancelled by the differential amplifier 210.

The structure of thermal couplers is explained next with reference toFIG. 5 which is a circuit diagram of the AC-DC converters 12 and 9respectively of the standard signal processing means and the outputsignal processing means. The AC-DC converter 212 of the standard signalprocessing means comprises not only two operational amplifiers 223 and224 but also two rectifying diodes 225 and 226. The AC-DC converter 209of the output signal processing means similarly comprises twooperational amplifiers 227 and 228 and two rectifying diodes 229 and230. These four operational amplifiers 223, 224, 227 and 228 may bearranged in a package 231 as shown in FIG. 6 and each pair of thesediodes 225 with 229 and 226 with 230 may be formed as a diode array 232,as shown in FIG. 7.

Although an example was shown for thermally coupling the AC-DCconverters 209 and 212, it now goes without saying that thermal couplingmay be effected between the amplifiers 208 and 211. Although exampleswere explained above for the measurement of the size or the shape of atarget object, they can also be used for measuring the displacement of adiaphragm and to thereby detect a pressure.

In summary, according to this invention, the amplitude of driving signalfor driving the differential transformer is controlled so as to takeupon a fixed value by means of a feed-back control. Thus, expensivecomponents with high stability are not required, unlike the prior artexamples relying upon an open-loop control, and detection ofdisplacements with high accuracy becomes possible even with the use ofrelatively inexpensive components.

Next, the structure of a displacement sensor embodying this invention isexplained with reference to FIG. 8 which is its exploded diagonal view,FIG. 9 which is its vertical sectional view, FIG. 10 which is asectional view of its tip part, FIG. 11 which is a sectional view of itscenter part, FIG. 12 which is a side view of its core shaft, and FIG. 13which is a sectional view of its backward part.

With reference to FIGS. 8-13, the displacement sensor has a tubularhousing 1 made of stainless steel having a plurality of stoppers 2formed around its peripheral surface 1A at equal intervals at its centerpart. As shown in FIG. 16, these stoppers 2 are formed as protrusions 3towards the center of the housing 1. These protrusions 3 may be made bya so-called punch-stretch forming method using a punch and a die (notshown). If the height of protrusion is x and the thickness of thehousing 1 is t, the housing 1 will easily develop a crack C as shown inFIG. 17 unless x<t/2. Thus, as shown in FIG. 16 by dotted lines, amaterial with thickness greater than t is punched by a distance of x andthen the outer surface is abraded by a centerless grinding process suchthat the unsightly indentations formed by the punching will become lessconspicuous. The front surface 3A of the protrusion 3 serves as thestopping surface, or contact surface.

As shown in FIGS. 9 and 13, a cable cap 74 integrally formed with acable 46 is affixed to the back end part of the housing 1. Polyesterelastomer may be used for forming the cable cap 74. A groove 76 isformed on the outer periphery of the cable cap 74 in the direction ofthe circumference for storing an adhesive agent inside. A protrusion 77is provided for sealing on the bottom surface of the groove 76 in thedirection of the circumference.

The cable cap 74 engages with the back part of the housing 1 with thegroove 76 filled with an adhesive F as shown in FIG. 13. The protrusion77 contacts the inner peripheral surface of the back part of the housing1 so as to seal in the adhesive F inside the groove 76. This serves toprovide a dependably watertight structure with improved tensilestrength.

As shown in FIG. 9, a linear bush 4, a stopper 5 for preventingrotation, a bobbin assembly 6A, a cable spacer 42 and abobbin-supporting spring 44 are placed inside the housing in this order.

As shown in FIGS. 9 and 14, the linear bush 4 has an outer tubular body7 containing therein a tubular ball guide 9 supporting many balls 8rotatably. The outer tubular body 7 has a tubular main body 7A with atube-attaching part 10 on the outer circumference which is lower thanneighboring parts by one step. At the center of this tube-attaching part10 is a hole 11 which is a substantially rectangular hole, as shown inFIG. 18, for stopping the rotation of the outer tubular body, elongatedin the axial direction of the outer tubular body 7 (indicated by adouble-headed arrow). The front and back ends 11 a and 11 b of the hole11 are perpendicular to the axial direction of the outer tubular body 7and the rounded corners of the rectangle of hole 11 have a small radiusof curvature. The hole 11 is thus rectangularly formed because if itsfront and back ends were semicircularly or elliptically formed, therotation-preventing pin 51A (to be described below) would get stuckagainst the inner wall, as shown in FIGS. 19A and 19B.

As shown in FIG. 9, a ring-shaped stopper 7B is provided on the innercircumference at the back of the outer tubular body 7. The end cap 12 ispressed into and thereby affixed to the front side of the outer tubularbody 7. A sealing groove 13 is formed on the end cap 12 and a back bootholder 14 is provided on the front side of the end cap 12.

As shown in FIG. 10, the ball guide 9 has a hole 15 at a positioncorresponding to the hole 11 in the outer tubular body 7 when the linearbush 4 is in the assembled condition. These two holes 11 and 15 areformed alike.

As shown in FIGS. 9 and 11, the stopper 5 comprises a main body 5A atthe back of which is formed a backward end part 16 having a smallerdiameter than the main body 5A. The inner surface side of the connectingpart 17 between the main body 5A and the backward end part 16 serves asa spring-supporting surface 18 provided with a stopper member 18A. Aplurality of flat parts 19 of a stopper 21 are formed on the outerperipheral surface on the backward side of the main body 5A at specifiedintervals, and a contact surface 20 is formed at the front end of theflat parts 19. When the stopper 5 is inserted into the housing 1, theprotrusions 3 of the stoppers 2 are at the positions of these flat parts19 of the opposite stopper 21 and the contact surfaces 3A and 20 are ina face-to-face relationship so as to position the stopper and to preventthe stopper from turning around.

The bobbin assembly 6A includes bobbin 24. A front shield-engaging part25 is formed at the front end of the bobbin 24. A back shield-engagingpart 26 and a housing-contacting part 27 having a larger diameter thanthe back shield-engaging part 26 are formed at the back end of thebobbin 24. A groove 28 for leading out a coil line is formed at twoplaces on the back shield-engaging part 26 and the housing-contactingpart 27. A partition 29 is formed on the center part of the bobbin 24,and a first coil 30 and a second coils 31 are wound in front of andbehind the partition 29. The bobbin 24 is covered with a shield member32 which is attached to the bobbin 24 with its front end placed over thefront shield-engaging part 25 and its back end placed over the backshield-engaging part 26 such that the first and second coils 30 and 31are covered. The shield member 32 is covered by a bobbin tube 33 made ofa thermocontracting synthetic resin film (such as a polyimide film).From the back part of the bobbin 24, first, second and third terminalpins 38, 39 and 40, connected to the connecting parts of the first andsecond coils 30 and 31 and a terminal at their ends, protrude backwardas shown in FIG. 8.

When the bobbin assembly 6A, thus formed, of the differentialtransformer 6 is contained and fastened inside the housing 1, the bobbinassembly 6A and the housing 1 are in a coaxial relationship. In thissituation, the backward end part 16 of the stopper 5 is pushed into thefront shield-engaging part 25 of the bobbin 24, the front end of thecable spacer 42 is contacting the back surface of a housing-contactingpart 41 at the back of the bobbin 24, and a bobbin-supporting spring 44is compressed between the back end of this cable spacer 42 and aspring-receiving part 43 at the tip of the cable cap 74 such that thebiasing force of this bobbin-supporting spring 44 serves to push thebobbin 24 against the stopper 5 through the cable spacer 42.

Thus, changes in the size of the inner components such as the stopper 5,the bobbin 24 and the cable spacer 42 are absorbed by thebobbin-supporting spring 44, preventing any gap from being generatedamong these inner components. Thus, the temperature coefficient and therepeatability improve.

The first and second coils 30 and 31 and the shield member 32 areconnected to the cable 46 through the wiring pattern on a flexiblecircuit board 45 positioned on the inner inside of the cable spacer 45.

A conic coil spring 48 and a mobile member 50 provided with a coremember 53 are inserted inside the housing 1. As shown in FIGS. 9 and 11,this mobile member 50 has a mobile shaft 51 and a core shaft 52 screwedinto a back end part of this mobile shaft 51. A core member 53 isattached to a back part of the core shaft 52. A female screw part 60 anda male screw part 78 are formed respectively at the front end part andthe back end p art of the mobile shaft 51 and a pin-accepting hole part79 is formed radially in the center part of the mobile shaft 51.

As shown in FIGS. 11 and 12, the core shaft 52 has a main body 52A onwhich a male screw part 54, a brim-shaped first stopper part 55, aspring support 56, a tapered part 57, a second stopper part 58 and aholder 59 holding the core member 53 are formed in this order from thefront side towards the back.

As shown in FIGS. 9, 10 and 15, a holder 61 for a measuring member 70has a main body 61A on which an attachment screw part 62, a stopper part63, a front boot holder 64 and a connecting screw part 65 formed in thisorder from the front side towards the back. A rubber boot 66 has a mainbody 66A on the front side of which a front seal part 67 is formed andon the back side of which a back seal part 68 is formed. The measuringmember 70 comprises a contact member 71 which is shaped like a ball anda ball holder 72 which supports the contact member 71 rotatably. Afemale screw part 73 is formed on the ball holder 72.

To assemble the linear bush 4, the ball guide 9 supporting many balls 8rotatably is placed inside the outer tubular body 7, therotation-preventing pin (also referred to as the “rotation-preventingmember”) 51A is inserted into the pin-accepting hole part 79 of themobile shaft 51 from the side of the hole 11 through the hole 15 of theball guide 9, and the tube-attaching part 10 of the outer tubular body 7is covered with the bobbin tube 10A such that the hole 11 is blocked bythermal contraction. As the linear bush 4 is engaged inside the housing1, its outer tubular body 7 becomes attached to the housing 1 by meansof an adhesive agent, and a means for preventing rotation of the mobilepart is formed with the holes 11 and 15, the rotation-preventing pin51A, and the pin-accepting hole part 79.

To assemble the mobile member 50, the male screw part 54 of the coreshaft 52 is screwed into the female screw part 78 at the back end partof the mobile shaft 51. A watertight O-ring 13A is provided in thesealing groove 13 formed on outer circumference of the end cap 12 andthe linear bush 4 is engaged with and fastened to the housing 1. Thewatertight ring 13A contacts the inner peripheral surface of the housing1 to form a reliable watertight contact. The mobile shaft 51 is thusheld by the linear bush 4 so as to be movable with respect to thehousing 1 in its axial direction. The back end part of the core shaft 52penetrates the backward end part 16 at the back of the stopper 5, andthe core member 53 of the core shaft 52 is inserted into the bobbin 24.The differential transformer 6 is formed with this core member 53 andthe aforementioned bobbin assembly 6A of the differential transformer 6.

The stopper 5 is prevented from turning around by engaging the frontsurface 3A of the protrusion 3 with the contact surface 20 of thestopper 21 to position the stopper 5 with respect to the housing 1 andpositioning the protrusions 3 of the housing 1 against the flat partsaround the stopper 5. The conic coil spring 48 has its tip part tocontact the spring support 56 on the core shaft 52 and its back end partto contact the spring-supporting surface 18 of the stopper 5 so as topush the mobile member 50 forward with its biasing force and to causethe front part of the mobile shaft 51 to protrude forward from the frontend of the housing 1.

The mobile member 50, the rubber boot 66 and the measuring member 70 areassembled by firstly engaging the front seal part 67 of the rubber boot66 to the front boot holder 64 of the holder 61 to thereby attach theholder 61 to the rubber boot 66, as shown by arrow (1) in FIG. 15, andthen engaging the connecting screw part 65 of the holder 61 with thefemale screw part 60 at the front end part of the mobile shaft 51, asshown by arrow (2). The back seal part 68 of the rubber boot 66 is thenengaged with the back boot holder 14 of the end cap 12 on the front sideof the outer tubular body 7, and the female screw part 73 of themeasuring member 70 with the attachment screw part 62 of the holder 61.

Next, the operation of the displacement sensor thus structured will bedescribed.

When the displacement sensor is set at a specified position and a mobiletarget object (not shown) is not contacting the contact member 71 of thesensor (at the “stretched limit position”), the mobile member 50 ispushed by the force of the conic coil spring 48 and the stopper part 55of the core shaft 52 engages with the stopper 7B on the innercircumference of the outer tubular body 7, as shown in FIG. 22A. In thissituation, the rotation-preventing pin 51A does not hit the front end 11a of the hole 11 or the front edge part 15 a of a hole 15 shown in FIG.22A.

When an AC current is supplied to the displacement sensor, currentsbegin to flow through the first and second coils 30 and 31 byelectromagnetic induction. If the core member 53 is at the center andequally over the first and second coils 30 and 31, the absolute valuesof the voltages generated in the first and second coils 30 and 31 arethe same.

If the target object is displaced, interferes with the contact member 71of the displacement sensor and pushes in the mobile member 50 againstthe force of the conic coil spring 48, the core member 53 supported bythe mobile member 50 is displaced backward inside the bobbin 24 of thedifferential transformer 6 from its center position, as shown in FIG.22B, so as to be inserted more deeply inside the second coil 31. Thus,the voltage induced in the second coil 31 becomes higher and the outputvoltage changes proportionally to the displacement of the core member53. The displacement of the target object can be determined by detectingthis change in the outputted voltage.

At the farthest pushed-in position, the a second stopper part 58 of thecore shaft 52 contacts the stopper member 18A on the spring-supportingsurface 18 of the stopper 5. At this moment, the rotation-preventing pin51A does not hit the back end 11 a of the hole 11 or the back edge part15 b of the hole 15. Thus, deformation of the rotation-preventing pin51A can be prevented, and since this rotation-preventing pin 51A doesnot become deformed, the core member 53 of the differential transformer6 is not displaced and the accuracy of detection is improved.

Merits of a displacement sensor thus structured will be explained next.

Firstly, since both the outer tubular body 7 and the ball guide 9 of thelinear bush 4 are provided with a hole for preventing rotation (shown at11 and 15), the mobile shaft 51 is provided with the pin-accepting holepart 79 for the rotation-preventing pin 51A, and the rotation-preventingpin 51A is inserted into these holes 11 and 15 in the direction ofmotion of the mobile member 50 and further into the pin-accepting holepart 79 so as to form a rotation preventing means, this means forpreventing rotation of the mobile member can be placed inside the mainbody of the sensor and hence the sensor can be made more compact and themain body can be made shorter.

Secondly, since these holes 11 and 15 are rectangularly elongated in thedirection of motion of the mobile member 15, the rotation-preventing pin51A does not get stuck in these holes and the mobile member 15 isallowed to move smoothly.

Thirdly, since the hole 11 is sealed with a thermocontracting resin tube10A, the adhesive used for attaching the linear bush 4 to the housing 1is dependably prevented from entering the holes 11 and 15.

Fourthly, since the rotation-preventing pin 51A does not hit the frontor back ends of the holes 11 and 15 even when the mobile member 50 is atthe “stretch limit position” with the conic coil spring 48 at the fullystretched position or at the farthest pushed-in position, deformation ofthe rotation-preventing pin 51A can be prevented and the displacement ofthe core member 53 of the differential transformer 6 can be eliminated.

Fifthly, since the core shaft 52 and the stopper 5 are respectivelyprovided with a spring support 56 and a supporting surface 18 for theconic coil spring 48 and the core shaft 52 is provided with the taperedpart 57 to prevent interference with the conic coil spring 48, the coniccoil spring 48 is prevented from hitting any neighboring components evenif the sensor is made compact and the required force of operation can bereduced.

Sixthly, since the mobile shaft 51 and the holder 61 for the measuringmember 70 are separate components, the outer tubular body 7 of thelinear bush 4 is provided with the end cap 12 having the back bootholder 14, the holder 61 is provided with the front boot holder 64, theholder 61 is attached to the rubber boot 66 by engaging the front endpart of the rubber boot 66 with the front boot holder 64, the holder 61is connected to the front end part of the mobile shaft 51 and the backend part of the rubber boot 66 is engaged with the back boot holder 14,the interior of the rubber boot 66 can be prevented from being damaged.

FIG. 27 shows a prior art contact displacement sensor for comparison.When this prior art sensor is assembled by engaging a rubber boot 123onto a mobile shaft 125 from the side of a measuring member 124, theinner seal surface 123 a of this rubber boot 123 is damaged by a malescrew part 126 on the mobile shaft 125. Thus, a reliable sealedcondition cannot be guaranteed with such a prior art sensor.

FIG. 20 shows in detail the end cap 12 provided to the outer tubularbody 7 of the linear bush 4 and the back boot holder 14 formed. Thus,the back seal part 68 of the rubber boot 66 can be engaged by this backboot holder 14, and the back seal part 68 can be kept sufficiently thickand a sufficient sealing distance d can be secured for making adependably waterproofed connection. This is to be contrasted with thecorresponding structure of a prior art sensor shown in FIG. 21 with theback boot holder 14-1 formed on the housing 1 such that the sealingdistance d′ at the back engaging portion 66′ of the rubber boot 66 wassmaller. Thus, the waterproofing was less dependable.

Further merits of the present invention include the use of thewatertight O-ring 13A provided in the sealing groove 13 formed on outercircumference of the end cap 12 such that the linear bush 4 is engagedwith and fastened to the housing 1.

Moreover, since the cable cap 74 is integrally formed with a cable 46with a synthetic resin material and is affixed to the back end part ofthe housing 1, the cable cap 74 is not a separate component and hencethe number of components and the production cost of the sensor can bereduced. The cable 46 can be made more flexible. Since a groove 76 isformed on the outer periphery of the cable cap 74 in the direction ofthe circumference for storing an adhesive agent inside and a protrusion77 is provided for sealing on the bottom surface of the groove 76 in thedirection of the circumference, an improved waterproofing is effected.

In summary, the present invention provides a compact displacement sensorwith a dependably waterproofed structure which can be produced easilyand at a reduced cost.

What is claimed is:
 1. A contact displacement sensor comprising: ahousing having inward protrusions with a stopping surface and definingan axial direction; a differential transformer of a half bridge typewith two coils having a core member; a linear bush inside said housing,said linear bush having an outer tubular body with a thermocontractingtube in said axial direction and a holder which is movable in said axialdirection; a mobile member having a two-stopper part and a mobile shaftsupporting said core member of said differential transformer, saidmobile member being movable in said axial direction, said mobile shafthaving a pin-accepting hole part; a contact member at a front end partof said mobile shaft; a spring outwardly biasing said mobile member;rotation-preventing means for preventing rotation of said mobile member,said rotation-preventing means including a rotation-preventing member,said outer tubular member and said holder each having a hole, saidrotation-preventing member being inserted movably in said axialdirection into the holes through said outer tubular member and saidholder and into said pin-accepting hole part.
 2. The displacement sensorof claim 1 wherein said outer tubular member and said holder each have asubstantially rectangular hole elongated in said axial direction.
 3. Thedisplacement sensor of claim 2 wherein the hole in said outer tubularmember is sealed with said thermocontracting tube.
 4. The displacementsensor of claim 3 wherein said housing has inward protrusions each witha stopping surface perpendicular to said axial direction; said housingincludes a stopper having an outer surface with flat parts and a contactsurface which is at one end of said flat parts and is perpendicular tosaid axial direction; and said protrusions are positioned at said flatparts and said contact surface and said stopping surface contact eachother to position said stopper and to prevent rotation of said stopper.5. The displacement sensor of claim 4 wherein said protrusions areproduced by punching said housing inward by a punch-stretch formingmethod, and grinding outer surface areas of said housing in a centerlessgrinding process to reduce the thickness of said housing whilemaintaining a specified distance of protrusion.
 6. The displacementsensor of claim 4 wherein said housing has a first stopping part and asecond stopper part affixed thereto; said mobile member has a firststopper part and a second stopper part; when said spring is at astretched limit position with the first stopper of said housing and thefirst stopper of said mobile member contacting each other, there is afinite interval between said rotation-preventing member and a front endpart of said holes; and when said spring is at a pushed-in position withthe second stopper of said housing and the second stopper of said mobilemember contacting each other, there is another finite interval betweensaid rotation-preventing member and a back end part of said holes. 7.The displacement sensor of claim 6 wherein the first stopper part of thehousing is formed on said outer tubular body, the second stopper part ofthe mobile member is formed on said stopper, said mobile shaft includesa core shaft; and the first stopper part and the second stopper part ofsaid mobile member are formed on said core shaft.
 8. The displacementsensor of claim 7 wherein said spring is a conic coil spring and issupported between said housing and said core shaft, and said core shaftincludes a tapered part for avoiding interference with said conic coilspring.
 9. The displacement sensor of claim 1 wherein said contactmember is supported by a contact-member holder; said mobile shaft andsaid contact-member holder are separate components; said outer tubularmember is provided with an end cap having a back boot holder; saidcontact-member holder includes a front boot holder; a rubber boot has afront end part engaged with said front boot holder and a back end partengaged with said back boot holder, said contact-member holder beingthereby attached to said rubber boot, and said contact-member holderbeing connected to said rubber boot; and said rubber boot has a back endpart engaged with said back boot holder.
 10. The displacement sensor ofclaim 9 wherein said end cap has a groove formed thereon; an O-ring isdisposed in said groove formed on said end cap, contacting an innersurface of said housing when said linear bushing is attached to saidhousing.
 11. The displacement sensor of claim 1 further comprising acable cap which is integrally formed with a cable connected to saiddifferential transformer, said cable cap engaging with said housing,said cable being pulled out of a back end part of said housing.
 12. Thedisplacement sensor of claim 11 wherein said cable cap has anadhesive-holding groove formed thereon; a sealing protrusion beinginside said adhesive-holding groove, said sealing protrusion contactingthe inner surface of said housing when said cable cap is engaged to saidhousing so as to seal in an adhesive inside said adhesive-holdinggroove.